This invention relates to semiconductor diodes, and, more particularly, to a modified double barrier tunnel diode.
A diode is a semiconductor device having a nonlinear voltage-current relation. Diodes are among the most important of solid-state devices, and are used widely in many electronic applications. The tunnel diode is one variety of diode, having the unusual characteristic of negative resistance. As the term is used, negative resistance is a voltage-current behavior wherein, over certain voltage ranges, increasing the voltage applied across the diode leads to decreased current carried through the diode. By contrast, in most devices an increasing applied voltage results in increasing current. Tunnel diodes have a number of applications, including high frequency oscillator circuits and high frequency electronic switches.
One type of tunnel diode is the double barrier tunnel diode. In an embodiment of particular interest, a double barrier tunnel diode includes a gallium arsenide quantum well having a thin barrier layer of aluminum gallium arsenide epitaxially joined on each side thereof. This structure, herein termed a quantum barrier, in turn lies between two injection layers of gallium arsenide. The quantum barrier creates an energy barrier to the flow of electrons that can be overcome by electrons only under certain conditions, and which results in the negative resistance characteristic of interest over a range of applied voltage. Electrons are injected into the quantum barrier from one of the injection layers under an internal voltage, produced by an applied external voltage. The internal voltage increases the energy of the injected electrons to satisfy the resonant tunneling condition of the quantum barrier. Under the proper conditions of voltage sufficient to satisfy the resonance condition, the electrons tunnel through the quantum barrier.
Not all of the conduction band electrons in the injection layer are able to pass through the quantum barrier, and the probability that an electron passes through as a function of electron energy is termed the transmission coefficient. Such double barrier tunnel diodes exhibit the negative resistance characteristic, since increasing voltages above the voltage required for satisfying the resonance condition reduce the transmission coefficient and thence the current flowing through the diode.
Conventional double barrier tunnel diodes have two problems that limit their usefulness in certain devices. First, a fairly high internal voltage, and consequently a high applied external voltage, is required to induce tunneling through the quantum barrier. (As will be discussed more completely below, the external or contact voltage is the voltage applied between the device contacts, while the internal voltage is that actually experienced across the quantum barrier.) Calculations and experiments have shown that the high internal voltage required for tunneling of the electrons creates an asymmetry in the potential profile that significantly reduces the transmission coefficient. That is, while the conventional double barrier tunnel diode does achieve the desired negative resistance effect, its efficiency in passing electrons at the resonance energy is significantly reduced from the theoretical maximum for a zero applied voltage. The high internal voltage required also implies a high DC operating point voltage to bias the device to its negative resistance operating range, when the double barrier tunnel diode is used as a component of an electronic circuit. The higher DC operating point voltage increases the power consumption of the device and also impairs the high frequency performance of the circuit.
Second, an important property used to describe the operation of the double barrier tunnel diode is the "peak-to-valley" ratio of the maximum current at the beginning of the negative resistance region to the minimum current at the end of the negative resistance region. The greater this ratio, the greater the operating range over which the device may be used in its negative resistance mode of operation, and the greater can be the output power of the device. The applied voltage required to produce tunneling in conventional double barrier tunnel diodes lowers this ratio, thereby decreasing the operating range of the diode.
For these reasons, the conventional double barrier tunnel diode is limited in some planned applications by the high internal voltage required, the high power consumption, and the low peak-to-valley ratio. There is a need for an improved device that achieves the negative resistance characteristic of the conventional double barrier tunnel diode, but does not have the previously discussed problems. The present invention fulfills this need, and further provides related advantages.